The plate load Z of the tube is fixed.? ? The 50 ohm output is fixed.? ?The PI net just matches the plate load Z to 50 ohms.? ?Knowing the typ load Q you want, the rest is easy....... as you can see on the GM3SEK PI spreadsheet.?
The tube can't source anything, it can only sink current.? In class AB2? (think SB-220, with it's 5vdc zener), the tube only conducts for 210 degs....and is cutoff the remaining? 150 degs.? ?To drive the grid positive, the cathode is driven negative.? ?Meanwhile the output is positive.? ?The input and output are in series....and 180 degs out of phase.....which is the reason that a tube in GG typ has 6db better IMD? vs the same tube that is grid driven.?
The input Z of the tube is sky high? during 150 degs....and low for 210 degs.? ?The PI tuned input provides for some flywheel action, and smooths out the overall input Z.?
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The tune and load cap, in some cases, is padded on the lower bands.? ?On my drake L4B amp, the load cap is padded on 80m.....(700 pf mica).? ?The load cap is a 4 x section air variable.? ? All 4 x sections are used on 40m only.? On 20-10m, only 2 of the 4 x sections are used.? ? ?The load cap on the drake does not have a 6:1 vernier......so by only using 2 of the 4 x sections of the load cap, the load control is less touchy.? ?The tune cap on the drake has a 6:1 vernier.....and tuning is not touchy at all.?
